1. Field of Invention
The embodiments of the present invention generally relate to a method and apparatus polishing a substrate in a chemical mechanical polishing system.
2. Background of Invention
In semiconductor wafer processing, the use of chemical mechanical planarization, or CMP, has gained favor due to the enhanced ability to increase device density on a semiconductor workpiece, or substrate, such as a wafer. Chemical mechanical planarization systems generally utilize a polishing head to retain and press a substrate against a polishing material while providing motion therebetween. Some planarization systems utilize a polishing head that is moved over a stationary platen that supports the polishing material. Other systems utilize other motions, for example, providing a rotating platen. A polishing fluid is typically disposed between the substrate and the polishing material during polishing to provide chemical activity that assists in the removal of material from the substrate. Some polishing fluids also contain abrasives.
One type of polishing material that may be utilized for chemical mechanical polishing is known as fixed abrasive polishing material. Fixed abrasive polishing material generally comprises a plurality of abrasive particles suspended in a resin binder that is disposed in discrete elements on a backing sheet. As the abrasive particles are contained in the polishing material itself, systems utilizing fixed abrasive polishing materials generally utilize polishing fluids that do not contain abrasives. Examples of fix abrasive polishing material are disclosed in U.S. Pat. No. 5,692,950, by Rutherford et al. (issued Dec. 2, 1997) and U.S. Pat. No. 5,453,312, by Haas et al. (issued Sep. 26, 1995), both of which are hereby incorporated by reference in their entireties.
FIG. 1 generally depicts a schematic of a conventional chemical mechanical polishing apparatus 100 that utilizes a web 102 of polishing material to process a substrate 116. The apparatus 100 generally includes at least one polishing station 106. The polishing station 106 includes a polishing platen 108 and a polishing head 110. The web 102 of polishing material is supported by the platen 108 below the polishing head 110. Generally, the platen 108 has a top surface 112 that supports a polishing area 114 of the web 102 where processing occurs. The substrate 116 is retained by the polishing head 110 and pressed against the polishing area 114 while being moved relative thereto during processing.
The polishing area 114 of the web 102 is generally held against the platen 108 during processing typically by tensioning the web 102 between a supply roll 118 and a take-up roll 120 that are disposed on opposite sides of the platen 108. The top surface 112 of the platen 108 may additionally contain a groove 122 that circumscribes the polishing area 114. The groove 122 is coupled to a vacuum source 124 so that air and other fluids that may be present between the web 102 and the platen 108 are evacuated through the groove 122, thus pulling the web 102 flush against the top surface 112 of the platen 108.
Generally, the web 102 includes a plurality of abrasive elements 130 disposed on a flexible backing 132. The abrasive elements 130 have a body 134 extending from the backing 132 and terminating in a working surface 136 that contacts the surface 128 of the substrate 116.
During the processing operation, a polishing fluid 126 is disposed on the web 102. The polishing fluid 126 generally provides chemical activity that assist in the removal of material from the surface 128 of the substrate 116 being polished. Optionally, the polishing fluid 126 may include abrasives to assist in the mechanical removal of material from the surface 128 of the substrate 116. Typically, polishing fluids 126 generally have a viscosity in the range of about 0.01 to about 1.0 centipoises.
A factor in robust polishing systems and processes is controlling the cost of consumables such as the web 102 of polishing material. One factor that is detrimental to web life is deformation of the abrasive elements during polishing. Excessive deformation of the abrasive elements causes instability in substrate to substrate polishing performance (i.e., rate, uniformity, defects and the like) and ultimately results in a requirement for higher usage rates of web material per wafer processed.
During CMP processing, the substrate 116 is typically pressed against the abrasive elements 130 of the web 102 with a force of about 1.5 to about 8 psi during polishing. The relative motion between the platen 108 and the polishing head 110 results in the substrate 116 having a velocity of about 200 to about 1000 mm/sec in relation to the web 102. The loading of the substrate 116 against the web 102 and shear forces created by the relative motion between the substrate 116 and web 102 result in the abrasive elements 130 being deformed. For comparison, an abrasive element 138 depicted in a non-deformed state is shown in phantom. The deformation of the abrasive elements 130 causes non-uniform wear of the elements 130. Over successive polishing cycles, the deformation of the abrasive elements takes on a permanent deformation set. The formation of a permanent deformation set within the field of abrasive elements further aggravates the non-uniform wear of the web 102 and additionally may weaken the elements 130 to the point where some elements 130 may detach from the backing 132, resulting in substrate scratching and web 102 failure. As such, deformed elements 130 substantially contribute to an undesirable rate of web consumption during polishing and poor polishing repeatability between substrates.
The effect of mechanical stresses causing undesirable deformation of the fixed abrasive elements is amplified by the effect of heat generated during the polishing process. Heat generated during the process of substrate polishing is partially absorbed by the web matrix material. The induction of heat into the web matrix material effectively reduces the relative modulus of the abrasive matrix features. In reducing the effective modulus of the fixed abrasive matrix features, the ability of the matrix material to withstand deformation under the applied mechanical stresses of the polish process is further reduced.
The polishing fluid 126 disposed within the process area of the web 102 generally provides little benefit in preventing deformation of the abrasive elements 130. Typically, the polishing fluid 126 is generally applied to the web 102 from a central location and flows across the polishing area 114 of the web 102. Due to the polishing fluids relatively low viscosity and wetting properties, as the polishing fluid 126 spreads across the web 102, the polishing fluid 126 does not completely surround the entire abrasive elements 130, particularly in the portion of the web 102 underneath the substrate 116. Additionally, air pockets 140 may form or be trapped between some of the abrasive elements 130 that underlie the substrate 116 thus displacing the polishing fluid 126 from completely wetting out and surrounding the abrasive elements 130.
In the absence of a more complete contact of the abrasive elements by the surrounding polishing fluid two important attributes to the polishing process are not realized. The limited interaction between the polishing fluid and the abrasive elements reduces the degree to which the process fluid can provide a heat sink and conduction path in reducing the latent heat build up within the abrasive elements. The ability to reduce the latent heat build up within the abrasive elements would limit the shear modulus loss that normally would be experience, reducing the level of deformation experienced, and in general provide improved process stability. Similarly, as the polishing fluid 126 does not completely surround the abrasive elements 130, there is an absence of fluid presented at the abrasive/substrate interface during polishing 126. In the absence of sufficient lubricity being provided between the substrate and abrasive elements, localized and excessive generation of heat during polishing may be realized causing an additional mechanism for mechanical instability of the abrasive elements.
Therefore, there is a need for a method and apparatus that improves the performance of polishing material.
In one aspect of the invention, an apparatus for polishing is provided. In one embodiment, an apparatus for polishing a substrate includes a polishing material having a fluid disposed thereon. The polishing material has a plurality of elements extending from a backing. The fluid fills the entire volume between the elements comprising the polishing material and has a viscosity between about 100 to about 10,000 centipoises.
In another aspect of the invention, a method for polishing is provided. In one embodiment, the method includes the steps of supporting a polishing material having abrasive elements disposed on a backing, disposing a substrate on the abrasive elements of the polishing material, providing a fluid on the polishing material wherein the fluid completely fills a volume defined between the substrate and the backing, and generating a hydrostatic force between the substrate and the backing.